How often do you eat tree bark? Every time you sprinkle cinnamon on your oatmeal or eat cinnamon rolls for breakfast. Cinnamon sticks are dried, curled up pieces of bark from Southeast Asian trees of the genus Cinnamomum.
The cork stoppers on wine bottles--also bark. Cork oaks have thick, corky bark that is harvested by stripping away the bark every decade without killing the tree, similar to birch bark but thicker.
While bark from certain species is useful to people, it is crucial for the survival of a tree. Bark prevents excess water loss, repels insects, protects the tree from temperature extremes and animals, and provides fire protection for certain species.
From white birch bark to the fibrous reddish strips of cedar, bark is as varied as the leaves on trees.
Unlike leaves, bark can look different with age. For example, trembling (or quaking) aspen has creamy-white bark with black markings after it grows out of its pale green juvenile bark. Then as the tree matures, the older bark at the base of the trunk can turn gray and become heavily fissured.
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| Young aspen tree |
To understand how bark becomes fissured with age on some trees, we have to go deeper into the tree. When looking at a stump, such as a ponderosa pine, there is a distinct line between the bark and the wood. Typically there is more wood than bark.
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| Bark is only a few inches thick on a grand fir over 150 years old |
When a tree grows, it grows from the cambium layer, which is a very thin layer of cells that encircles the tree between the bark and the wood. The cambium layer produces cells to the inside that creates the sapwood and cells to the outside that creates bark.
The cells created to the outside of the cambium layer are phloem cells, which carry nutrients manufactured by the leaves down to the roots. New cells are created each year, which pushes the oldest phloem cells to the outside. The old cells eventually die and don’t transport nutrients.
Holding all the phloem cells in is a layer of cork cambium. The cork cambium creates cork cells towards the outside of the tree which is the bark visible on a tree.
As the tree grows in girth, something has to give, especially since there are dead cells that cannot stretch. What gives are the cork cells. As the new growth pushes outward, the cork cells crack and form fissures and ridges. Thus, fissures form on trees past a certain diameter, which depends on the tree species.
Since the base of the tree is the oldest, it has the thickest bark and biggest fissures. Thick bark is beneficial to trees, particularly those that grow in fire-prone areas. Sequoia have bark that can be two feet thick. Thick bark often has huge fissures and ridges that create updrafts which carries heat up and away from the trunk.
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| The thick bark on a ponderosa pine protects the tree's living tissues from fire |
Part of the bark’s protective nature comes from the cork. As the cork cells mature they deposit a waxy material called suberin in their walls and die. The suberin helps the bark be waterproof and air-proof.
Some species’ cork cells are so air-proof that the inside of the tree can’t breathe. The cambium layer and the cells it creates need air from the outside for cellular respiration. To counteract the airtight bark, the tree creates lenticels, which are the horizontal lines on bark, such as on birch. The lenticels act like windows in the bark, allowing for air exchange.
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| Bark peeling off birch trees |
As a tree matures, the oldest bark can be sloughed off. The ground around the base of a mature ponderosa pine is often covered with puzzle-like pieces of bark sloughed off as the tree grows.
Animals, such as beavers, moose and porcupines, tend to eat younger bark since it isn't as tough as older bark. They also eat the nutritious cambium layer underneath the bark. No matter how many animal's depend on eating bark to survive the winter, I'll only eat bark in the form of cinnamon, particularly in cinnamon rolls.
Note: Published in the Bonners Ferry Herald on Feb. 14, 2013.
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